Method for the preparation of stabilized sodium percarbonate

ABSTRACT

The invention relates to a method for increasing the storage stability of sodium percarbonate (2 Na 2  CO 3 .3 H 2  O 2 ) by coating with a sodium perborate. 
     According to the present invention wet sodium percarbonate salt, obtained by reacting sodium carbonate with hydrogen peroxide in aqueous phase and separating the salt from the aqueous phase, is treated with a solution or suspension containing sodium perborate in a solid-liquid separator by a kind of displacement wash. In contrast to previously known methods, according to the present invention a greater storage stability is attained with smaller quantities of perborate coating.

DESCRIPTION

The invention relates to a method for the preparation of stabilisedsodium percarbonate consisting of a sodium percarbonate core and acoating containing sodium perborate of the formula NaBO₂.H₂ O₂.nH₂ O,wherein n is 0, 1, 2 or 3. The method according to the present inventionallows good storage stability to be attained, notwithstanding a smallquantity of coating material.

Sodium percarbonate (2 Na₂ CO₃.3 H₂ O₂) is used as an active oxygencomponent in washing agents, bleaches and cleaning materials. Owing toits unsatisfactory storage stability in a warm, damp environment and inthe presence of various washing agent components and cleaning materialcomponents, sodium percarbonate must be stabilised. An essentialprinciple of stabilisation is to surround the particles of sodiumpercarbonate with a coating of stabilising components.

Thus the coating of sodium percarbonate with paraffin or polyethyleneglycol is well-known. However adequate long-term stability is notachieved through this and besides the solubility in water is reduced inan undesirable manner. The formation of an alkali silicate coat on theparticles of sodium percarbonate, as is proposed in DE-OS 26 52 776,also results in inadequate stabilisation and also to an undesirableproportion of insoluble constituents. According to the methods knownfrom DE-OS 24 17 572 or DE-OS 26 22 610, sodium sulphate and sodiumcarbonate, or sodium sulphate, sodium carbonate and sodium silicate areused as coating components, wherein the methods are based on spraying asolution of the coating components onto particles of sodium percarbonatein a fluidised bed drier. For stabilisation in practice a quantity ofcoating material of at least 3% by weight, but in most cases definitelyin excess of this, is required.

While the use of a coating material containing at least one boroncompound from the group comprising metaboric acid, orthoboric acid andtetraboric acid for the stabilisation of sodium percarbonate is knownfrom DE-PS 28 00 916, the stabilising action attainable thereby is shownin the comparative examples given in DE-OS 33 21 082 to be inadequateand in contrast to that sodium percarbonate with a coating containingsodium borate, in particular sodium metaborate, is presented in thatpatent as being advantageous. As the inventors of the presentapplication established when reworking the examples from DE-OS 33 21082, in order to achieve an adequate stability the coating had tocontain borate in a quantity such that the boron content of the sodiumpercarbonate stabilised in this way amounted to at least 0.4% by weight,however the available active oxygen content of the stabilised sodiumpercarbonate was consequently each time clearly less than 14% by weight.A further development in stabilisation using borates is described inEP-A 0 487 256, however the method disclosed therein is in two steps,with a drying stage being required after each step; hence the method istechnically expensive.

Finally, DE-AS 24 58 326 discloses a method for stabilising sodiumpercarbonate, wherein the storage stability of the pure product, as wellas mixed with cleaning materials which accelerate decomposition, isincreased. Here the sodium percarbonate is coated with a hydrophobicliquid organic compound to which sodium perborate powder is added.

The disadvantage of this method is the necessity of using a hydrophobicliquid organic compound, which, optionally, for the purpose of betterhandling must be diluted with a lower alcohol. Moreover the quantitiesof coating chemicals used, namely 5 to 20% by weight of sodium perborateand 5 to 10% by weight of hydrophobic organic compound, calculatedrespectively by reference to sodium percarbonate, are very high.

Sodium percarbonate layer particles, which consist of a sodiumpercarbonate core and a coating of sodium perborate, wherein the sodiumperborate contains less than 3 moles of water of crystallisation permole, are known from DE-PS 26 51 442; according to DE-PS 27 12 139, thecoating may contain in addition sodium silicate and other hygroscopicsubstances. To prepare the said sodium percarbonate layer particles,sodium percarbonate is first of all wetted with water or with an aqueoussodium silicate solution in a quantity too small for the formation ofsodium perborate tetrahydrate to be attained and then coated withdehydrated sodium perborate. A further development of the previouslyoutlined method may be quoted from DE-PS 28 10 379: here sodiumpercarbonate is sprayed with an aqueous solution of sodium perboratecontaining from 50 to 500 g of sodium perborate tetrahydrate per 1 ofsolution at 40° to 60° C. and a solution of sodium silicate, and thenthe water introduced is wholly or partially removed again. As shown bythe example in DE-PS 28 10 379, it is not possible to spray the wholerequired quantity of solution containing perborate onto thecentrifuge-damp sodium percarbonate all at once and then to dry it.Rather the solution must be sprayed in several portions, each time withintermediate drying, which considerably impairs the economic efficiencyof the method.

The object underlying the present invention is the provision of animproved method for the preparation of stabilised sodium percarbonatehaving a coating containing sodium perborate which avoids thedisadvantages of the typical methods previously known. The method ofapplying the coating containing sodium perborate should be capable ofbeing integrated without difficulty with previously known methods forthe preparation of sodium percarbonate by reacting sodium carbonate withhydrogen peroxide in aqueous phase. A further object is directed toattaining as good a stability as possible with as small a quantity aspossible of perborate in the coating.

A method has been found for the preparation of stabilised sodiumpercarbonate consisting of a sodium percarbonate core and a coatingcontaining sodium perborate of the formula NaBO₂.H₂ O₂.nH₂ O, wherein nis 0, 1, 2 or 3, by the spraying of a wet sodium percarbonate salt,produced in a manner known per se by reacting sodium carbonate withhydrogen peroxide in aqueous phase and separation from the latter, withan aqueous solution or suspension containing sodium perborate, thesodium perborate content of which, expressed as NaBO₂.H₂ O₂, is from 15to 450 g/l, and drying at 40° to 90° C., characterised in that the wetsodium percarbonate salt is sprayed with 5 to 100 1 of the solution orsuspension containing sodium perborate per 100 kg of the wet salt, andexcess solution or suspension is separated simultaneously orsubsequently in a solid-liquid separator and that, in the case of asuspension containing sodium perborate being used, suspended sodiumperborate particles have essentially a smaller particle diameter thanthe sodium percarbonate particles.

Although NaBO₂.H₂ O₂.nH₂ O is frequently encountered in the literatureas a notation for sodium perborate, in reality it relates to substancesof the formula ##STR1## wherein m=0, 2, 4 or 6. m=0 corresponds toNaBO₂.H₂ O₂, which is generally referred to as sodium perboratemonohydrate; m=6 corresponds to NaBO₂.H₂ O₂.3H₂ O, which is generallyreferred to as sodium perborate tetrahydrate.

The method according to the present invention follows directly on thepreparation of sodium percarbonate according to a so-called wet processat the stage of the wet salt, so that only one drying stage is necessarywithin the overall process. Well-known wet processes for the preparationof sodium percarbonate comprise the reaction of sodium carbonate withhydrogen peroxide in aqueous phase, the crystallisation of the sodiumpercarbonate thus formed, the separation of the latter from the aqueousphase (mother liquor) and the drying of the wet sodium percarbonatesalt. For the preparation of a wet sodium percarbonate salt referencemay be made, for example, to DE-PS 28 00 760 and to the documents citedtherein. Since all production plants operated according to a wet processalways include a device for solid-liquid phase separation and the samedevice can also be used for the treatment according to the presentinvention, only small adjustments are required to adapt an existingplant for the preparation of sodium percarbonate by the wet process sothat stabilised sodium percarbonate according to the present inventioncan be prepared.

While in typical previously known methods (DE-PS 28 10 379) the requiredquantity of spraying solution containing sodium perborate was sprayed inseveral portions and each one was dried, in the method according to thepresent invention the application of the perborate-containing coating iscarried out in the form of a so-called displacement wash. The motherliquor from the crystallisation process adhering to the wet sodiumpercarbonate salt is wholly or partially displaced by the solution orsuspension containing perborate by separating off excess liquid by meansof a solid-liquid separator after, or preferable during, spraying withthe solution or suspension. Through this displacement wash, theparticles of sodium percarbonate are brought into contact evenly withthe solution or suspension containing sodium perborate, so that acomplete coating of the particles is possible. At the same time a highstorage stability of the coated particles is ensured, although only asmall quantity of coating material, by reference to sodium percarbonate,is required to form the coating.

The solution or suspension containing sodium perborate which is used forspraying contains sodium perborate in dissolved form and, in the case ofa suspension, in suspended form. The solution preferably to be used,wherein the presence of a small quantity of extremely fine particles isnot excluded, is preferably a supersaturated perborate solution. Such asolution can be obtained by bringing together, immediately prior tospraying, an aqueous solution of sodium metaborate and an aqueoussolution of hydrogen peroxide in a molar ratio of approximately 1: 1.The two components may be reacted in a reactor or in a static mixer. Ata higher concentration of the NaBO₂ solution (above 3 mol/l) somewarming is necessary (40° to 60° C.); the concentration of the H₂ O₂solution is preferably 50 to 70% by weight. Since the reaction(formation of perborate) proceeds exothermically, the temperature rises;under these conditions there is generally no precipitate, which isadvantageous in view of the spraying of the reaction solution in thedisplacement wash.

In the event of a suspension containing suspended sodium perborate beingused, the diameter of the particles of the suspended sodium perborate,which in this case is present as sodium perborate tetrahydrate (NaBO₂.H₂O₂.3H₂ O), should essentially be smaller than the diameter of those ofthe sodium percarbonate to be coated. In this connection, by the term"essentially" it is understood that the particle diameter of at least90% of the sodium percarbonate is greater than the particle diameter ofthe suspended sodium perborate. The diameter of the suspended particlesof sodium perborate is preferably very much smaller than the diameter ofthe particles of sodium percarbonate to be coated. In this manner partof the suspended sodium perborate is precipitated during thedisplacement wash onto the surface of the sodium percarbonate while theremainder, together with the excess solution, is separated off in thesolid-liquid separator. As commercial sodium percarbonate preferably hasa grain size essentially greater than 150 μm, it is suitable to use asthe spraying solution a suspension containing perborate the particlesize of which is smaller than 100 μm. The solution or suspensionseparated off in the displacement wash may again be used as a sprayingsolution or suspension after adjusting the desired sodium perborateconcentration.

The quantity to be used of aqueous solution or suspension containingsodium perborate and their concentration depend on the stabilisationrequired. It is particularly preferred to treat wet sodium percarbonatesalt, the moisture content of which is generally between 5 and 12% byweight, with 10 to 50 1 of spraying solution per 100 kg of wet salt,wherein the content of sodium perborate, expressed as NaBO₂.H₂ O₂, inthe suspension is 200 to 400 g/l.

The required displacement wash according to the present invention may infact take place in a separate solid-liquid separator, for example acentrifuge or filtering device, but it is however more advantageous toperform the displacement wash in the same apparatus in which the wetsodium percarbonate salt is separated off from the mother liquor of thecrystallisation. Here, after maximum separation of the mother liquor,the wet salt is sprayed with the solution or suspension containingperborate; then subsequently, or preferably at the same time, excesssolution or suspension is removed by means of the solid-liquidseparator. A particularly advantageous embodiment of the invention is touse a screen-conveyor centrifuge or a push-type centrifuge fitted with awashing device as the solid-liquid separator.

Centrifuges of this type generally have at least two compartments,wherein the separation of the wet sodium percarbonate salt from themother liquor takes place in the first compartment and the displacementwash and removal of the excess solution or suspension takes place in thesecond.

The suspension or solution containing sodium perborate which is used inthe displacement wash may also contain, in addition to sodium perborate,other stabilising coating materials already known from prior art. Butsodium perborate is preferably the main component in the sprayingsolution or suspension. In so far as the solution or suspension removedduring the displacement wash is reused following adjustment to thedesired sodium perborate content, constituents of the mother liquor fromthe preparation of the wet sodium percarbonate salt are also present inthis solution. It is of advantage to use, together with the solution orsuspension containing perborate, a small quantity of known active oxygenstabilisers like, for example, phosphonic acid compounds capable ofcomplex formation, such as were used for example in DE-PS 37 20 277, toreduce the tendency of sodium percarbonate to reagglomerate. While itwas necessary in typical previously known methods also to add sodiumsilicate to the solution containing sodium perborate, in the methodaccording to the present invention this is unnecessary. Hence an alkalisilicate-free solution or suspension containing sodium perborate ispreferably used, because both the rate of dissolving and the dissolvingto give a clear solution of the stabilised sodium percarbonate areaffected in an undesirable manner by the presence of alkali silicate inthe coating.

The drying of the sodium percarbonate treated with the sodium perboratesolution or suspension takes place in a manner known per se, forexample, in fluidised bed driers. During the drying process thetemperature is preferably operated so that the sodium perboratetetrahydrate (NaBO₂.H₂ O₂.3H₂ O) present on the particles of sodiumpercarbonate is melted. The melting point of sodium perboratetetrahydrate is in fact 64° C.; however it was established that meltingprocesses are already occurring clearly below this temperature becausethe sodium perborate tetrahydrate is not present in the coating in apure form. The drying temperature is preferably maintained at leasttemporarily above the melting point of the sodium perboratetetrahydrate, so that partial or complete dehydration of thetetrahydrate to the so-called monohydrate (NaBO₂.H₂ O₂) occurs.

The formation of a uniform coating and hence an effective stabilisationare achieved both through the displacement wash and through the meltingprocess during drying. Surprisingly, the stability of the sodiumpercarbonate is considerably increased even with a very small quantityof coating material consisting essentially of sodium perborate, in factperborate equivalent to a quantity of from 0.1 to 0.2% by weight ofboron by reference to the stabilised sodium percarbonate. Even in thepresence of a phosphate-free, zeolite-containing washing tower powder ofa type usual on the market, stabilised sodium percarbonate according tothe present invention has a high residual active oxygen content evenafter prolonged storage under extreme conditions. Through the methodaccording to the present invention it was possible to dispense with theintermediate drying stages necessary in previously known methods; at thesame time the boron content in the coat could be further reduced andconsequently the active oxygen content of the stabilised sodiumpercarbonate increased. The method according to the present inventioncan be integrated without difficulty into existing plants for theproduction of sodium percarbonate by a wet process and requires only alittle extra technical expenditure, namely a facility for thepreparation of the solution or suspension containing sodium perborateused in the displacement wash and for recycling of the same.

EXAMPLE 1 AND COMPARATIVE EXAMPLES VB1 TO VB5.

Wet sodium percarbonate salt was prepared in an operation according tothe process described in DE-PS 28 00 760. After reacting sodiumcarbonate with hydrogen peroxide in a mother liquor containing kitchensalt and sodium hexametaphosphate and crystallising the sodiumpercarbonate, the latter was separated as far as possible from themother liquor as a wet salt in the first compartment of a two-chamberedscreen-conveyor centrifuge.

Flow of sodium carbonate suspension 2600 kg/h with a solid content of16% by weight, inclination of the drum 20°, centrifugal numberapproximately 1000 revs/min, differential speed between conveyor anddrum 20 revs/min, mesh size of the sieve 0.15 mm, residual moisture ofthe wet sodium percarbonate salt after leaving the first compartmentapproximately 10% by weight.

Above the second compartment of the centrifuge the wet salt was sprayedwith an aqueous solution by means of a single-component nozzle--forcontents of the spraying solution, concentration and volume flow, seeTable 1--and simultaneously dehydrated by centrifuging. The centrifugeproduct from the second compartment was recycled after restoring thecontent of the coating substances to the initial value by furtheraddition of the appropriate substances.

The product removed from the centrifuge, which had a residual dampnessof 7% by weight, was dried in a multistage fluidised bed drier with thetemperature profile in the fluid bed rising from 40 to between 80° and85° C.; the product was then cooled.

To assess the stability, samples of sodium percarbonate mixed with acommercial phosphate-free but zeolite-containing washing tower powder(Persil Supra TP)--proportions of mixture 15 to 85--in closed packets(0.4 1) were stored in a climatic test cabinet at a constant temperatureof 30° C. and 80% relative humidity. The results of the active oxygencontent of mixtures of sodium percarbonate, prepared according to thepresent invention (Example 1) and for comparison (VB1 to VB5),determined in the usual manner initially and after 4 and 8 weeks instorage, are shown in Table 2.

                  TABLE 1                                                         ______________________________________                                                           Active oxygen content                                                         (Oa) (%) (by reference                                     Contents and             to the sodium percar-                                concentration   Quantity bonate in the mixture)                                       of aqueous  of spray After After After                                Examples                                                                              solution    (l/h)    mixing                                                                              4 wks 8 wks                                ______________________________________                                        VB1     No washing  75       14.0  8.1   4.2                                  VB2     Washing only                                                                              75       14.1  7.7   5.1                                          with water                                                            VB3     MgSO.sub.4.7H.sub.2 O                                                                     75       13.9  7.8   4.3                                          (400 g/l)                                                             VB4     Hydroxyethane-                                                                            75       14.0  8.5   5.2                                          diphosphonic                                                                  acid                                                                          (75 g/l)                                                              VB5     Na-hexameta-                                                                              75       14.1  9.0   6.0                                          phosphate                                                                     (350 g/l)                                                             Sample  Sodium      75       14.3  11.6  8.8                                  according                                                                             perborate                                                             to      (410 g/l,                                                             Example 1                                                                             reported as                                                                   NaBO.sub.2.H.sub.2 O.sub.2)                                           ______________________________________                                         Key: VB = Comparative example                                            

In Example 1, a suspension containing suspended sodium perborate wasused, wherein the particle size was essentially less than 0.1 mm; apartfrom sodium perborate, the suspension contained only the constituentsconditional on the method dissolved in the mother liquor. Sodiumpercarbonate stabilised according to the present invention has aconsiderably higher residual oxygen content after storage than thecomparative products not processed according to the present invention.

EXAMPLES 2 to 7

The tests were carried out as in Example 1. The sodium perborateconcentration and the volume flow of the spray solution (suspension)were varied; volume flow of the wet salt was as in Example 1. Thestability of mixtures in storage was analysed (as described above). Theboron content of the stabilised sodium percarbonate was also analysed.

The good stabilisation is attributed to the fact that during thedisplacement wash all the sodium percarbonate particles in thecentrifuge, regardless of particle size or distribution, are in a thinlayer uniformly in contact with the coating components, which are atleast partially dissolved. In previously known methods using a mixer orin fluidised bed treatment, there is only a certain statisticalprobability of the particles of sodium percarbonate coming into contactwith the coating components; therefore with these methods a largerquantity of coating substance is usually necessary in order to obtain astabilising effect equal to that of the method according to the presentinvention.

                  TABLE 2                                                         ______________________________________                                                                       Oa content (%)                                               Concentra-       (By reference                                  Exam- Vol-    tion (calcu-                                                                             Boron to the sodium per-                             ple   ume     lated as   content                                                                             carbonate in the mixture                       num-  Flow    NaBO.sub.2.H.sub.2 O.sub.2                                                               (% by After After After                              ber   (l/h)   in g/l)    wt)   mixing                                                                              4 wks 8 wks                              ______________________________________                                        2      30     30.3       0.06  14.05 9.2   5.8                                3      75     30.3       0.12  14.15 9.5   7.2                                4      75     41.0       0.18  14.05 11.5  8.8                                5     150     30.3       0.14  14.30 10.2  8.1                                6     200     30.3       0.14  14.20 10.1  8.0                                7     200     41.0       0.20  14.20 11.4  8.6                                ______________________________________                                    

We claim:
 1. A method for the preparation of stabilized sodiumpercarbonate formed of a sodium percarbonate core and a coatingcontaining sodium perborate of the formula NaBO.sub. 2.H₂ O₂.nH₂ O,wherein n is 0, 1, 2 or 3 comprising spraying a wet sodium percarbonatesalt with 5 to 100 1 of the solution or suspension containing sodiumperborate per 100 kg of the wet salt, the sodium perborate content ofwhich, expressed as NaBO₂.H₂ O₂ is from 15 to 450 g/l, and separatingexcess solution or suspension simultaneously or subsequently from solid.2. The method according to claim 1 wherein said wet sodium percarbonatesalt is prepared by reacting sodium carbonate with hydrogen peroxide inaqueous phase and separating the resulting product from said aqueousphase.
 3. The method according to claim 1 further comprisingsubsequently drying at 40° to 90° C.
 4. The method according to claim 1further comprising that, in the case of a suspension containing sodiumperborate being used, suspended sodium perborate particles haveessentially a smaller particle diameter than the sodium percarbonateparticles.
 5. The method according to claim 1, wherein the wet sodiumpercarbonate salt is sprayed with an aqueous solution or suspensioncontaining sodium perborate, in a quantity of from 10 to 50 1 per 1000kg of wet salt.
 6. The method according to claim 5 wherein said wetsodium percarbonate salt is sprayed with an essentially particle-freesolution, the sodium perborate content of which is expressed as NaBO₂.H₂O₂.
 7. The method according to claim 6 wherein said sodium perboratecontent is approximately 200 to 400 g/l.
 8. The method according toclaim 1 wherein a centrifugal action is used for solid-liquid separationand spraying and separation of excess solution or suspension essentiallytakes place simultaneously.
 9. The method according to claim 1 whereindrying is carried out at a temperature at least temporarily above themelting point of sodium perborate of the formula NaBO₂.H₂ O₂. 3H₂ O,wherein the crystallisation water is at the same time partially orcompletely removed.
 10. The method according to claim 1 wherein from 0.1to 0.2% by weight of boron based on the weight of sodium percarbonate isdeposited on said core.
 11. The method according to claim 1 wherein nointermediate drying stage is carried out.
 12. The method according toclaim 1 wherein the diameter of sodium perborate in a suspension ofparticles thereof is smaller than the diameter of sodium percarbonateparticles.
 13. The method according to claim 1 wherein an alkalisilicate free sodium borate is used.
 14. The method according to claim 1further comprising drying said sodium percarbonate after spraying withsaid sodium borate at a temperature sufficient to melt sodium perboratetetrahydrate so as to obtain a coating of sodium perborate as NaBO₂.H₂O₂.
 15. A method for the preparation of stabilized sodium percarbonateformed of a sodium percarbonate core and a coating containing sodiumperborate of the formula NaBO.sub. 2.H₂ O₂.nH₂ O, wherein n is 0, 1, 2or 3 comprising spraying a wet sodium percarbonate salt with 5 to 100 1of the solution or suspension containing sodium perborate per 100 kg ofthe wet salt, the sodium perborate content of which, expressed asNaBO₂.H₂ O is from 15 to 450 g/l, and separating excess solution orsuspension simultaneously or subsequently from solid, wherein saidseparating is carried out by a centrifuge.
 16. The method according toclaim 15 wherein said centrifuge is a screen conveyor or push typecentrifuge and takes place simultaneously.
 17. A method for thepreparation of stabilized sodium percarbonate comprising forming asodium percarbonate core by reacting sodium carbonate with hydrogenperoxide in aqueous phase, crystallizing the resulting sodiumpercarbonate to obtain a wet sodium percarbonate having mother liquoradhering thereto, spraying said wet sodium percarbonate salt with 5 to100 liters of a solution or suspension containing sodium perborate per100 kg of said wet salt, the sodium perborate content of which,expressed as NaBO₂.H₂ O₂ is from 15 to 450 g/l, displacing the motherliquor adhering to said wet sodium percarbonate salt by said solution orsuspension of sodium perborate so as to completely coat said sodiumpercarbonate with a coating containing sodium perborate of the formulaNaBO₂.H₂ O₂.nH₂ O, wherein n is 0, 1, 2 or
 3. 18. The method accordingto claim 17 wherein said displacing is carried out with a solid-liquidseparator after or during spraying with said solution or suspension. 19.The method according to claim 17 further comprising carrying out saidspraying in the same solid-liquid separator where the mother liquor isdisplaced from the sodium percarbonate salt.